Cells, Genes & Molecules Taxonomy & Evolution

Can you find genetic echoes of an ancient ancestor in hornworts?

The genes hornworts share with other living plants may tell us something about their ancient common ancestor.

Hornworts are unassuming plants, often found in damp corners. Many, gardeners consider them to be a weed. They are part of the bryophytes (which also includes mosses and liverworts) and are possibly the earliest diverging land plants. All land plants derive from an unknown common ancestor, descending from this point like a family tree. Hornworts are currently believed to be the closest still-existing group of plants to this ancestor. This means that hornworts may be able to give an idea of what some of the earliest land plants looked like, and how the jump between multicellular algae and land plants occurred. They have therefore fascinated evolutionary biologists for a long time. Their relatively simple body plan and restriction to damp environments is consistent with their ancient origins but appearances can be deceptive, which is why the recently assembled genomes of three species of the hornwort genus Anthoceros published in Nature Plants (here and here) will be important to understanding more about how plants made it onto the land and later produced the diversity that we see today.

Anthoceros. Image: Peter Pearsall / U.S. Fish and Wildlife Service.

It is often suggested that whole genome duplications have been key in plant evolution to build the diversity of plants we see today, as they may allow accumulation of large numbers of genes involved in building body plans. Consistent with this, the new Anthoceros genomes show no signs of having undergone a whole genome duplication and have low numbers of genes involved in building the plant body plan. Interestingly, while gene families involved in building elaborate body plans are unexpanded in the hornwort genome, other gene families are expanded and populous in the hornwort genome. In one of the species sequenced, Anthoceros angustus, these include gene groups believed to be involved in DNA repair, desiccation tolerance and synthesis of UV-protective pigments. These traits possibly reflect adaptation to the harsh environments early land plants had to contend with.

Genomes representing the two other members of the bryophytes, mosses and liverworts, have already been assembled and show significant amounts of horizontal gene transfer (i.e. transfer of genes from one species into another) into these groups of plants from bacteria and fungi. One of the new genomes from the species Anthoceros angustus confirms that this also a feature of the hornworts, and the authors detect 14 genes derived from horizontal transfer. Hornworts and the other bryophytes probably acquired such genes through the intimate symbioses they form with bacteria and fungi. Many of the genes acquired horizontally by hornworts are putatively annotated as being stress response genes. The authors speculate that such methods of acquiring genes in the bryophytes gave them the flexibility needed to adapt to the harsh conditions of life on land. One symbiosis of vast importance to land plants is the association between many species of plants and arbuscular mycorrhizal fungi. Two of the hornwort genomes presented possess all the important genes for this symbiosis that are known from flowering plants, indicating that the genetic components required for this crucial interaction between plants and fungi were present in the earliest days of land plants, and may have been important in the transition of plants to land.

While a lot can be inferred from a genome, many things cannot be proven just on this basis. The assembly of the first full hornwort genomes from three Anthoceros species will, however, provide a very useful platform for future work on this and other early-diverging land plants. In particular, it will assist with working towards one of the holy grails of molecular and cell biology studies in any species, which has not yet proved possible in hornworts: genetic tractability (the ability to manipulate genomes). Hornworts are much more than just damp-loving weeds.